As shown in FIG. 1, the compressor section 10 of a turbine engine is enclosed within an outer casing 12. The compressor can include a rotor (not shown) with a plurality of axially spaced discs 14. Each disc 14 can host a row of rotating airfoils, commonly referred to as blades 16. The rows of blades 16 alternate with rows of stationary airfoils or vanes 18. The vanes 18 can be provided as individual vanes, or they can be provided in groups such as in the form of a diaphragm. The vanes 18 can be mounted in the compressor section 10 in various ways. For example, one or more rows of vanes 18 can be attached to and extend radially inward from the compressor shell 12. In addition, one or more rows of vanes 18 can be hosted by a blade ring or vane carrier 20 and extend radially inward therefrom.
The compressor section 10 contains several areas in which there is a gap or clearance between the rotating and stationary components. During engine operation, fluid leakage through clearances in the compressor section 10 contributes to system losses, making the operational efficiency of a turbine engine less than the theoretical maximum. FIG. 2 shows three areas in which flow leakage can occur. First, leakage can occur across a clearance 22 between the tips 24 of the rotating compressor blades 16 and the surrounding stationary structure, such as the outer casing 12 or the vane carrier 20. Second, there are clearances 26 between one or more compressor seals 28 provided on the vanes 18 and a portion of the rotating compressor disc 14. Third, for cantilevered type vanes 18, there can be a clearance between the tips of the compressor vanes 18 and the substantially adjacent rotating structure (not shown).
Small clearances are desired to keep air leakage to a minimum; however, it is critical to maintain a clearance between the rotating and stationary components at all times. Rubbing of any of the rotating and stationary components can lead to substantial component damage, performance degradation, and extended outages. The size of each of the compressor clearances change during engine transient operation due to the difference in the thermal inertia of the rotor and discs 14 compared to the thermal inertia of the stationary structure, such as the outer casing 12 or the vane carrier 20. Because the thermal inertia of the vane carriers 20 are significantly less than the rotor, the vane carrier 20 has a faster thermal response time and responds (through expansion or contraction) more quickly to a change in temperature than the rotor.
Many prior efforts have focused on the avoidance of blade tip rubbing. For instance, large tip clearances 22 are initially provided so that the blade tips 24 do not rub during non-standard engine conditions where the clearances 22 would otherwise be expected to be the smallest. Examples of such non-standard operating conditions include hot restart (such as, restarting the engine soon after shutdown), spin cool, etc. However, because the minimum tip clearances 22 are sized for these off design conditions, the clearances 22 become overly large during normal engine operation, such as at base load. Consequently, the compressor and the engine overall can experience measurable performance decreases in power and efficiency due to tip clearance leakage.
Other prior approaches for addressing the tip rubbing issue have included providing abradable coatings 30 on the vane carriers 20 or other stationary structure. The abradable coatings 30 are made of a material that is softer than the compressor blades 16, so the tips 24 of the blades 16 can rub against the coating 30 without being damaged. However, in the end, the abradable coating 30 is rubbed away, resulting in a larger tip clearance 22 than is desired during normal operation. In addition, the spallation of the coating 30 has resulted in large patches of uncoated surfaces. Again, all of the above approaches only address the blade tip clearance 22 and not the other compressor clearances previously discussed.
Thus, there is a need for a system that can improve engine performance by controlling the various compressor clearances.